Combustors provided with variable dome valves

ABSTRACT

A combustor is provided having a variable dome valve on the upstream end of the flame tube thereof for controlling admission of a stream of combustion air to said flame tube.

This application is a division of copending application Ser. No. 567,954filed Apr. 14, 1975, now U.S. Pat. No. 4,049,021.

This invention relates to end or dome valves and to combustors having aflame tube provided with a said valve.

Air pollution has become a major problem in the United States and otherhighly industrialized countries of the world. Consequently, the controland/or reduction of said pollution has become the object of majorresearch and development effort by both governmental and nongovernmentalagencies. Combustion of fossil fuel is a primary source of saidpollution. It has been alleged, and there is supporting evidence, thatthe automobiles employing conventional piston-type engines burninghydrocarbon fuels are a major contributor to said pollution. Vehicleemission standards have been set by the U.S. Environmental ProtectionAgency (EPA) which are sufficiently restrictive to cause automobilemanufacturers to consider employing alternate engines instead of theconventional piston engine.

The gas turbine engine is being given serious consideration as analternate engine. CO emissions in conventional prior art gas turbineprocesses operated for maximum fuel combustion efficiency are notusually a problem. However, nitrogen oxides emissions, usually referredto as NO_(x), are a problem because the high temperatures generated insuch prior art processes favor the production of NO_(x). A gas turbineengine employed in an automobile or other vehicle will be operated overa wide range of varying operating conditions including a driving cyclecomprising idle, low speed, moderate speed, high speed, acceleration,and deceleration operations. These varying conditions also createserious problems in controlling both NO_(x) and CO emissions.Frequently, when a combustor is operated for the control of one ofNO_(x) or CO emissions, control of the other is lost. Both must becontrolled. Thus, there is a need for a combustor of practical and/orrealistic design, which can be operated in a manner such that thepollutant emissions therefrom will meet said EPA standards. Even acombustor, and/or a combustion process, giving reduced pollutantemissions approaching said standards would be a great advance in theart. Such a combustor, or process, would have great potential valuebecause it is possible the presently very restrictive EPA standards maybe relaxed even further than has been recently indicated.

The present invention solves the above described problems by providing adome valve which is adapted to be mounted on the dome or upstream end ofthe flame tube in a combustor. Said dome valve provides means forvarying and/or controlling the flow of a stream of combustion air into afirst combustion region of a combustor in accordance with fuel flow tosaid combustor. Such control makes possible the operation of a combustorover a wide range of driving conditions, such as the above-describeddriving cycle, and still maintain control of both NO_(x) and COemissions.

In its broadest aspects the valve of the invention is not limited tobeing employed on a flame tube of a combustor. As described furtherhereinafter, the valve of the invention can also be employed on the endof a conduit for controlling fluid flow through said conduit.

Thus, according to the invention, there is provided a valve, adapted tobe mounted on the end of a tubular conduit for controlling fluid flowthrough said conduit, said valve comprising: a fixed tubular innermember, closed at one end and open at the other end; a plurality ofopenings provided at spaced apart locations around and extending throughthe tubular wall of said inner member adjacent said closed end thereof;an adjustable annular throttle member rotatably mounted on and aroundsaid tubular member; a plurality of openings provided around andextending through said throttle member, said openings being of a size, ashape, a number, and located at spaced apart locations, corresponding tosaid openings in said tubular inner member; mounting means secured tosaid tubular inner member adjacent and around said open end thereof formounting said valve on the end of said conduit; and means for rotatingsaid throttle member around said inner tubular member within limits soas to bring said openings in said throttle member into and out ofregister with said openings in said tubular member and thereby vary theeffective size of said openings.

In a presently preferred embodiment the invention provides a variabledome valve, adapted to be mounted on the dome end of a flame tube in acombustor for controlling flow of a stream of combustion air into saidflame tube, said valve comprising: a fixed generally cylindrical innermember, closed at one end and open at the other end; a plurality ofopenings provided at spaced apart locations around and extending throughthe cylindrical wall of said inner member adjacent said closed endthereof; a mounting flange secured to and around said inner memberadjacent said open end thereof for mounting said valve on a said flametube; an adjustable throttle ring rotatably mounted on and around saidcylindrical inner member; a plurality of openings provided around andextending through said throttle ring, said openings being of a size, ashape, a number, and located at spaced apart locations, corresponding tosaid openings in said inner member; and means for rotating said throttlering around said inner cylindrical member so as to bring said openingsin said throttle ring into and out of register with said openings insaid inner member and vary the effective combined open area of saidopenings and thereby control the flow of a stream of combustion air intosaid flame tube.

FIG. 1 is a view, partially in cross section, of a combustor having avariable dome valve mounted on the flame tube of the combustor inaccordance with the invention.

FIG. 2 is an enlarged view, in elevation, taken along the line 2--2 ofFIG. 1 and illustrating one set of tangential entry ports or slots inthe flame tube of the combustor.

FIG. 3 is an enlarged view, in elevation, taken along the line 3--3 ofFIG. 1 and illustrating another set of tangential entry ports or slotsin the flame tube of the combustor.

FIG. 4 is a diagrammatic perspective view, partially cut away, of theupstream end of the combustor of FIG. 1 showing the flame tube and thevariable dome valve, and further illustrating certain operationalfeatures thereof.

FIG. 5 is a perspective view further illustrating an element of the domevalve of the combustor of FIG. 1.

FIG. 5A is a perspective view illustrating a modification of the elementillustrated in FIG. 5.

FIG. 6 is a perspective view further illustrating another element of thedome valve of the combustor of FIG. 1.

FIG. 6A is a perspective view illustrating a modification of the elementillustrated in FIG. 6.

FIG. 7 is a sectional view taken along the line 7--7 of FIG. 1.

FIG. 7A is a top plan view illustrating a modification of the valve ofthe invention.

FIG. 8 is a sectional view, taken through a location corresponding tothat of FIG. 7, and illustrating features of another dome valve whichcan be employed in combustors in accordance with the invention and inthe operation of said combustors.

FIG. 9 is a top plan view of the downstream portion of the flame tube ofthe combustor of FIG. 1.

FIG. 10 is a sectional view taken along the line 10--10 of FIG. 9.

Referring now to the drawings, wherein like or similar referencenumerals are employed to denote like or similar elements, the inventionwill be more fully explained with particular reference to preferredembodiment thereof.

FIGS. 1-7, inclusive, 9, and 10 illustrate a combustor provided with avariable dome valve in accordance with the invention. Said combustor isdenoted generally by the reference numeral 10. Preferably, saidcombustor comprises an outer housing or casing 12 having a flame tube 14disposed, preferably concentrically, therein and spaced apart from saidcasing to form an annular chamber 16 between said casing 12 and saidflame tube 14. Said flame tube can be supported in said housing orcasing by any suitable means. While it is preferred to provide thecombustor with an annular casing or housing, similarly as illustrated,so as to provide said annular space 16 for supplying air to the variousinlets (described hereinafter) in said flame tube, it is within thescope of the invention to alter the configuration of said housing orcasing, or to omit said housing or casing and supply said air inletsindividually by means of individual conduits. Said flame tube 14 isprovided at its dome or upstream end with a dome valve 18. A fuel inletmeans is provided for introducing a stream of fuel into the upstream endportion of said flame tube. As illustrated in FIG. 1, said fuel inletmeans comprises a fuel conduit 44 leading from a source of fuel andextending into communication with fuel nozzle 24 mounted in fuel flange22 which closes the upstream end of casing 12. Said fuel nozzle extendsinto said dome valve 18. An annular orifice means is disposed on thedownstream side of said dome valve 18. Said orifice means can preferablybe formed integrally with said dome valve as here illustrated and canpreferably comprise an annular flange 94 for mounting the downstream endof said dome valve 18 onto the upstream end of said flame tube 14. Afirst orifice 95 formed in said orifice means can be considered todefine the outlet from said dome valve 18 and the inlet to the firstcombustion section of the combustor.

A variable first air inlet means is provided in said dome valve foradmitting a variable volume of a first stream of air through said domevalve, around said fuel inlet nozzle 24, and into said first combustionregion 27 of said flame tube. As described further hereinafter, saidvariable first air inlet means comprises at least one air passage meansof variable cross-sectional area provided in and extending through saiddome valve 18 into communication with said first combustion region 27,and means for varying the cross-sectional area of said air passage meansand thus controlling the volume of said first stream of air admitted tosaid first combustion region. A second air inlet means is disposed inthe wall of said flame tube for tangentially admitting a second streamof air into said first combustion region 27 tangential to the wallthereof. Said second air inlet means preferably comprises a plurality oftangential slots 28 extending through the wall of the upstream endportion of said flame tube 14 at a first station in the flame tubeadjacent said outlet from said dome valve 18. A third air inlet means isdisposed in the wall of said flame tube downstream from said second airinlet means for tangentially admitting a third stream of air into asecond combustion region 31 located in said flame tube 14 adjacent,downstream from, and in communication with said first combustion region27. Said third air inlet means preferably comprises a plurality oftangential slots 30 extending through the wall of an intermediateportion of said flame tube 14 at a second station in the flame tubeadjacent and downstream from a second orifice 29 which can be consideredto define the outlet from said first combustion region. A third orifice32 is disposed in said flame tube adjacent and downstream from saidtangential slots 30. Preferably, a fourth air inlet means, comprising atleast one opening 34, is provided in the wall of said flame tube at athird station downstream from said third air inlet means 30 and saidthird orifice 32 for admitting a fourth stream of air comprising quenchor dilution air into said flame tube 14.

Said flame tube 14 can be fabricated integrally if desired. However, forconvenience in fabrication, said flame tube can preferably be formedwith its wall divided into separate sections similarly as hereillustrated. Thus, in one preferred embodiment said tangential slots 28can be formed in an upstream first wall section 36 of said flame tube,preferably in the upstream end portion of said first wall section withthe downstream wall of said flange 94 forming the upstream walls of saidslots. In said preferred embodiment said tangential slots 30 can beformed in an intermediate second wall section 38 located adjacent anddownstream from said first wall section 36. Preferably, said second wallsection 38 is disposed with its upstream edge contiguous to thedownstream edge of said first wall section 36, and said tangential slots30 are formed in the upstream end portion of said second wall section 38with the downstream edge of said first wall section 36 forming theupstream walls of said slots 30. In this preferred embodiment said thirdorifice 32 is formed in said second wall section 38 and adjoins saidslots 30 formed therein. Preferably, the inner wall surface of saidfirst wall section 36 tapers inwardly from the downstream edge of saidtangential slots 28 to the upstream edge of said second orifice 29 toform an inwardly tapered passageway from said slots to said orifice.Preferably, the downstream end of said second wall section 38 comprisesan annular radially extending wall member 33, with said third orifice 32being formed in said wall member 33 and with the upstream wall of saidwall member 33 comprising at least a portion of the downstream walls ofsaid slots 30. Said annular wall member 33 provides for the abruptexpansion of hot combustion products flowing from first combustionregion 27 to second combustion region 31.

It will be understood that the combustors described herein can beprovided with any suitable type of ignition means and, if desired, meansfor introducing a pilot fuel to initiate combustion. For example, aspark plug 37 can be mounted to extend through flange 22 and theupstream end of dome valve 18 as shown.

Referring to FIGS. 4, 5, 6, and 7, said dome valve 18 comprises a fixed,preferably generally cylindrical, inner member 80 (see FIG. 6) closed atone end and open at the other end. A plurality of openings 82 areprovided at spaced apart locations around the circumference of saidcylindrical member 80 adjacent the closed end thereof. An opening 84 isprovided in said closed end for receiving a fuel inlet nozzle, e.g.,nozzle 24 of FIG. 1, which extends through the flange 22 of housing orouter casing 12. The outlet of said fuel nozzle would be positionedsimilarly as shown for nozzle 24 in FIG. 1. Said fuel inlet nozzle canbe any suitable type of fuel nozzle. As here shown it is an air assistfuel nozzle of conventional design wherein air is used in atomizing thefuel. Another opening 88 is provided in said closed end for receiving anigniter means, such as spark plug 37 in FIG. 1, which also extendsthrough said flange 22. Openings 92 are provided for receiving mountingbolts (not shown) for mounting the dome valve on said flange 22 andwithin housing or casing 12. Preferably, a mounting flange 94 isconnected to and provided around the open end of said cylindrical member80 for mounting said member 80 on the upstream end of a flame tube,e.g., flame tube 14 in FIG. 1. Any other suitable mounting means can beprovided on the base of said member 80. Preferably, a groove 96 isprovided in said flange 94 around the open base of said cylindricalmember 80. A pair of spaced apart stop pins 98 project from said flange94 perpendicular thereto and adjacent said cylinder member 80. Anorifice 95, preferably tapered inwardly, is provided in said flange 94adjacent and in communication with the open end of said cylindricalmember 80. Thus, said flange 94 comprises an orifice means with saidorifice 95 defining the outlet from said dome valve.

The adjustable throttle ring 100 of FIG. 5 is mounted around saidcylindrical inner member 80 and is provided with a plurality of spacedapart openings 102 therein of a size, number, and shape and at spacedapart locations, corresponding to said openings 82 in cylindrical member80. Said throttle ring fits into groove 96 in flange 94 when said grooveis provided. An actuator pin 104 projects outwardly from the outersurface of said throttle ring 100 and coacts with said stop pins 98 tolimit the movement of said ring 100. Friction reducing lugs 106 can beprovided on the top and the bottom of said ring 100 for movably bearingagainst the inner surface of flange 22 in housing 12 and the bottom ofgroove 96, respectively, FIG. 7 is a cross section of ring 100 mountedon member 80.

FIG. 8 illustrates a modified cylindrical member 80' which can beemployed in a modification of said dome valve 18. Said modifiedcylindrical member 80' is essentially like the cylindrical member 80shown in FIGS. 6 and 7 except that openings 82' in the modifiedcylindrical member 80' extend tangentially therethrough instead ofradially. It will be understood that the corresponding openings in thecorresponding modified throttle ring (not shown) which is employed withsaid modified cylindrical member 80' are correspondingly tangential.

It has been found that when combustors are provided with air assist fuelinlet nozzles, or with any other air assist fuel introduction means, itis desirable to control the amount of air supplied to the fuel nozzle inaccordance with the fuel flow to said nozzle. Any suitable control meanscan be employed for this purpose and the specific means illustrated inFIG. 1 forms no part, per se, of this invention and can be modified orsubstituted for as desired. As shown diagrammatically in FIG. 1, theflow controller 114 actuates valve 116 in air conduit 118 responsive tothe flow of fuel through the orifice in fuel conduit 44 to program anincrease in air flow to nozzle 24 to accompany an increase in fuel flow,or vice versa. Said valve 116 can be a flow control valve forcontrolling volume of flow, or a pressure regulator valve for holding aconstant pressure in the conduit downstream therefrom.

Further, it has been found that when combustors are provided withvariable dome valves, such as dome valve 18 in FIGS. 1 and 4, it isdesirable to control the effective open area of the air inlet openingsin said dome valve in accordance with fuel flow to the combustor. Anysuitable control means can be provided for this purpose and, referringnow to FIG. 4, the specific means there illustrated forms no part, perse, of the present invention and can be modified or substituted for byany means known in the art. As shown diagrammatically in FIG. 4,controller 109, responsive to the flow of fuel through the orifice infuel conduit 44, actuates linkage 110, which is operatively connected tothe outboard end portion of control rod 111, and programs rotation ofsaid control rod in one direction or the other. Yoke member 112 is fixedto the inboard end of rod 111 inside of housing 12. The U-shaped recessin one end of yoke member 112 coacts with actuator pin 104 to causerotation of throttle ring 100 within the limits of the space betweenstop pins 98 and thus adjust the effective size of the opening providedby openings 82 and 102. As here shown, said openings 82 and 102 are indirect register with each other to provide the maximum opening into domevalve 18. Indicator pin 113 is provided to indicate the degree ofrotation of throttle ring 100. A knob (not shown) can be provided on theoutboard end of control rod 111 for manual operation, if desired.

The openings provided in said cylindrical inner member 80 and saidthrottle ring 100 have been illustrated as circular in shape. However,it is within the scope of the invention for said openings to berectangular in shape, e.g., square, as shown by opening 102' in FIG. 5Aand opening 82" in FIG. 6A. An advantage of square openings is that theamount of air admitted through the openings is directly proportional tothe rotation of the throttle ring in varying the size of said openings.With circular openings the amount of air admitted through said openingsis not proportional to said rotation. A further advantage is theincrease in area of a square opening, e.g., about 28 percent for a 0.75inch square opening over a 0.75 inch diameter circular opening.

Said inner member 80 is illustrated as being cylindrical, and saidthrottle ring 100 has been illustrated as circular. This is preferred.However, it is within the scope of the invention for the interior ofsaid inner member to be tubular, but not necessarily cylindrical.

In one method of operating the combustor of FIG. 1, a first stream ofair is introduced through dome valve 18 at a controlled rate into firstcombustion region 27 of the combustor. In the combustor of FIG. 1 saidfirst stream of air is introduced generally radially with respect tosaid first combustion region. A stream of fuel is introduced, preferablyaxially, into said first combustion region 27. In one embodiment, saidfuel is sprayed into said first combustion region as a hollow cone andsaid first stream of air is introduced around the stream of fuel andintercepts said cone. The rate of introduction of said first stream ofair is controlled in accordance with the rate of introduction of saidstream of fuel, as described elsewhere herein.

A second stream of air is tangentially introduced into said firstcombustion region 27 via tangential slots 28 in a direction tangentialthe wall of said first combustion region. Said slots 28 thus impart aswirl to said second stream of air. The direction of said swirl can beeither clockwise or counter-clockwise. When employing the slotsillustrated in FIG. 2 the direction of swirl will be clockwise, lookingdownstream in the flame tube. Said first and second streams of air forma combustible mixture with said fuel, and at least partial combustion ofsaid mixture is caused in said first combustion region. Hot combustionproducts and any remaining said mixture are passed from said firstcombustion region 27, through orifice 29, and into second combustionregion 31.

A third stream of air is tangentially introduced into said secondcombustion region via tangential slots 30 in a direction tangential thewall of said second combustion region. Said slots 30 thus impart a swirlto said third stream of air. The direction of swirl imparted to saidthird stream of air can be either clockwise or counter-clockwise, but ispreferably opposite the direction of swirl imparted to said secondstream of air by said slots 28. When employing the slots illustrated inFIG. 3 the direction of swirl of the third stream of air will becounter-clockwise, looking downstream of the flame tube. Said thirdstream of air surrounds said hot combustion products and any remainingmixture entering from the first combustion region, and mixes therewith.Combustion is essentially completed in said second combustion region.

Preferably, a fourth stream of air is introduced via openings 34 andmixes with combustion products leaving said second combustion region.Said fourth stream of air comprises quench or dilution air. The hotcombustion gases then exit the combustor to a turbine or otherutilization.

A number of advantages are realized in the practice of the invention.The combustors described herein are low emission combustors. Saidcombustors are small compact combustors which are particularly wellsuited to be employed in locations where space is important, e.g., underthe hood of an automobile. Yet, the principles involved and the advancesprovided by the invention are applicable to combustors employed inlarger power plants, e.g., large stationary gas turbine engines,boilers, etc. The variable dome valves employed in combination with theflame tubes in the combustors described herein contribute to the overallefficiency of the combustors. Said variable dome valve is located in arelatively cool low stress region of the combustor, i.e., at theupstream end of the flame tube. Said variable dome valve is a smallcomponent comprising only one movable element which operates with only asmall movement from a closed position to an open position. Thus, rapidresponse to changing operating conditions is provided. This combinationof a variable dome valve with relatively small flame tubes renders thecombustors particularly well suited for mobile installations. Incontrast, the "variable hardware" of the prior art combustors usuallyprovides for adjustments at a plurality of locations in the combustors,including adjustments to the hot flame tube itself. The result isusually a large, bulky, unit which in practical operation functionspoorly, if at all.

EXAMPLE

Test runs carried out in combustors having configurations essentiallylike that of the combustor illustrated in FIG. 1, and calculations basedon the data obtained in said test runs, have demonstrated the advantagesof employing the valve of the invention on the flame tube of combustors.Said test runs and data included operation of the combustors undercomparable conditions wherein: (a) the effective size of the openings indome valve 18 were varied in accordance with fuel flow to the combustor;and (b) the effective size of the openings in dome valve 18 wasmaintained fixed at about 9.9 percent of the total flame tube openings(dome valve openings plus other flame tube openings). Said test runswere carried out over a simulated driving cycle consisting ofdeceleration, idling (from 0 to less than about 20 miles per hour), lowroad load (from about 20 to about 40 miles per hour), high road load(greater than about 40 miles per hour), and acceleration. The resultsshowed there was a decrease in the production of NO_(x) emissions in theorder of 80 to 85 percent when varying the effective size of the domevalve openings in accordance with fuel flow, as compared to operatingwith the dome valve openings fixed at said 9.9 percent open. Saiddecrease in NO_(x) production was accomplished with complete control ofCO emissions production at a level less than present EPA standards.

By way of further illustration, but not by way of limitation, theabove-described inner member 80 of dome valve 18 in FIG. 1 had an insidediameter of 3.500 inches and an outside diameter of 4.000 inches. Saidflange 94 had an outside diameter of 5.5 inches. Said throttle ring 100had an inside diameter of 4.008 inches and an outside diameter of 4.5inches. The openings in said inner member 80 and said throttle ring 100were 0.750 inches in diameter.

As mentioned above, while the valve of the invention has been describedwith particular reference to being employed as a dome valve on the domeor upstream end of the flame tube in a combustor, the valve can beemployed in other locations. In general, said valve can be employed onthe end of other conduits for controlling flow of a fluid through saidconduits. In such instances the valve would preferably be provided witha cover plate 99 secured to the closed end of inner member 80 by meansof suitable mounting bolts 91 extending into openings 92 in said closedend as shown in FIG. 7A. Said cover plate would preferably extend to theouter edge of throttle ring 100 in such instances.

The term "air" is employed generically herein and in the claims toinclude air and other combustion supporting gases.

Thus, while certain embodiments of the invention have been illustratedand described for illustrative purposes, the invention is not limitedthereto. Various other modifications or embodiments of the inventionwill be apparent to those skilled in the art in view of this disclosure.Such modifications or embodiments are within the spirit and scope of thedisclosure.

We claim:
 1. A combustor comprising a flame tube disposed in an outercasing to provide an annular space between said flame tube and saidcasing, and a variable dome valve mounted on the upstream end of saidflame tube, and wherein said dome valve comprises:a fixed generallycylindrical inner member, closed at one end and open at the other end; aplurality of openings provided at spaced apart locations around andextending through the cylindrical wall of said inner member adjacentsaid closed end thereof; a mounting flange secured to and around saidinner member adjacent said open end thereof for mounting said valve onsaid flame tube; an adjustable throttle ring rotatably mounted on andaround said cylindrical inner member; a plurality of openings providedaround and extending through said throttle ring, said openings being ofa size, a shape, a number, and located at spaced apart locations,corresponding to said openings in said inner member; and means forrotating said throttle ring around said inner cylindrical member so asto bring said openings in said throttle ring into and out of registerwith said openings in said inner member and vary the effective combinedopen area of said openings and thereby control the flow of a stream ofcombustion air into said flame tube.
 2. A combustor comprising a flametube disposed in an outer casing to provide an annular space betweensaid flame tube and said casing, and a variable dome valve mounted onthe upstream end of said flame tube, and wherein said dome valvecomprises:a fixed generally cylindrical inner member, closed at one endand open at the other end; a plurality of openings provided at spacedapart locations around and extending through the cylindrical wall ofsaid inner member adjacent said closed end thereof; a mounting flangesecured to and around said inner member adjacent said open end thereofand having a groove therein adjacent and around the base of said innermember for mounting said valve on said flame tube; an adjustablethrottle ring seated in and rotatably mounted in said groove to rotateon and around said cylindrical inner member; a plurality of openingsprovided around and extending through said throttle ring, said openingsbeing of a size, a shape, a number, and located at spaced apartlocations, corresponding to said openings in said inner member; a pairof spaced apart stop pins extending perpendicularly from said flange andadjacent the outer wall surface of said cylindrical inner member; andmeans for rotating said throttle ring around said inner cylindricalmember, so as to bring said openings in said throttle ring into and outof register with said openings in said inner member and vary theeffective combined open area of said openings and thereby control theflow of a stream of combustion air into said flame tube, including anactuator pin extending radially from the outer wall surface of saidthrottle ring at a location between said stop pins.
 3. A combustoraccording to claim 2 wherein:a fuel flange closes one end of said outercasing; said closed end of said inner member of said dome valve, mountedon the dome or upstream end of said flame tube, is secured to said fuelflange; and said means for rotating said throttle ring comprises: arotatable control rod extending through the wall of said casing; a yokemember, having a U-shaped recess formed in one end thereof, mounted onthe inboard end of said control rod with said actuator pin of saidthrottle ring disposed in said recess; and means connected to theoutboard end of said control rod for rotating said control rod.
 4. Acombustor according to claim 2 wherein:said tubular inner member isgenerally cylindrical in shape; and said throttle member is circular inshape.
 5. A combustor according to claim 4 wherein:said openings in saidcylindrical inner member are circular in shape and extend radiallytherethrough; and said openings in said circular throttle member arecircular in shape and extend radially therethrough.
 6. A combustoraccording to claim 4 wherein:said openings in said cylindrical innermember are circular in shape and extend tangentially therethrough; andsaid openings in said circular throttle member are circular in shape andextend tangentially threthrough.
 7. A combustor according to claim 4wherein:said openings in said cylindrical inner member are rectangularin shape and extend radially therethrough; and said openings in saidcircular throttle member are rectangular in shape and extend radiallytherethrough.
 8. A combustor according to claim 4 wherein:a plurality offriction reducing lugs extend from the bottom surface of said circularthrottle member at spaced apart locations; and in operation, said lugsmovably bear against the bottom of said groove in said flange.
 9. Acombustor comprising a flame tube disposed in an outer casing to providean annular space between said flame tube and said casing, and a variabledome valve mounted on the upstream end of said flame tube, and whereinsaid dome valve comprises:a fixed generally cylindrical inner member,closed at one end and open at the other end; a plurality of openingsprovided at spaced apart locations around and extending through thecylindrical wall of said inner member adjacent said closed end thereof;an adjustable throttle ring rotatably mounted on and around saidcylindrical inner member; a plurality of openings provided around andextending through said throttle ring, said openings being of a size, ashape, a number, and located at spaced apart locations, corresponding tosaid openings in said inner member; and means for rotating said throttlering around said inner cylindrical member, so as to bring said openingsin said throttle ring into and out of register with said openings insaid inner member and vary the effective combined open area of saidopenings and thereby control the flow of a stream of combustion air intosaid flame tube.